In the shoulder-girdle the coracoids are large and meet in a ventral symphysis; precoracoids and a sternum are apparently absent, but parts generally regarded[71] as the clavicles and interclavicle are well developed. In the pelvis, the pubes and ischia meet in a long symphysis. The limbs are pentedactylate, and in the best known forms, the Plesiosauridae, form swimming paddles. The Sauropterygia occur in beds of Secondary age, and some of the best known genera are _Plesiosaurus_, _Pliosaurus_ and _Nothosaurus_. _Order 3._ CHELONIA. In the Tortoises and Turtles the body is enclosed in a bony box, formed of the dorsal carapace, and a flat ventral buckler, the plastron. Except in _Dermochelys_ the carapace is partly formed from the vertebral column and ribs, partly from dermal bones. Both carapace and plastron are, except in _Dermochelys_, _Trionyx_ and their allies, covered with an epidermal exoskeleton of horny plates, which are regularly arranged, though their outlines do not coincide with those of the underlying bones. The thoracic vertebrae have no transverse processes, and are quite immovably fixed, but the cervical and caudal vertebrae are very freely movable. There are no lumbar vertebrae. The skull is extremely solid, and frequently has a very complete false roof. Teeth have been detected in embryos of _Trionyx_ but with this exception the jaws are toothless, and are encased in horny beaks. The quadrate is firmly fixed. The facial part of the skull is very short, and the alisphenoidal and orbitosphenoidal regions are unossified. In living forms there are no separate nasal bones, while large prefrontals and postfrontals are developed. There is a comparatively complete bony palate chiefly formed of the palatines and pterygoids. The anterior nares are united and placed at the anterior end of the skull, and the premaxillae are very small. There is no transpalatine bone and the vomer is unpaired. The dentaries are generally fused together. There are ten pairs of ribs, and each rib has only a single head and is partially attached to two vertebrae; there are no cervical or sternal ribs. There is no true sternum. The three anterior elements of the plastron are respectively homologous with the interclavicle and two clavicles of other reptiles, while the remaining elements of the plastron are probably homologous with the abdominal ribs of Crocodiles. The pectoral girdle lies within the ribs, and the precoracoids and coracoids do not meet in ventral symphyses. The scapula and precoracoid are ossified continuously. The pubis probably corresponds with the pre-pubis of Dinosaurs. There are four limbs each with five digits. The order includes three suborders:-- _Suborder (1)._ TRIONYCHIA. The carapace and plastron have a rough granular surface covered with skin and without any horny shields.